1. Field of the Invention
The present invention relates to a rolling bearing for use in a spindle for machine tools which operates at a high rotary speed or the like.
2. Description of the Related Art
A bearing for spindle of machine tools is required to be excellent in properties such as vibration resistance and acoustic properties for the purpose of enhancing the precision in machining. The bearing for spindle of machine tools is also required to undergo lubrication with grease, which can be easily handled and advantageous from the environmental and economical standpoints of view, operate at a high rotary speed and exhibit a prolonged lifetime.
In general, grease-lubricated rolling bearings to be incorporated in the spindle of machine tools are lubricated only with the grease which has been initially filled therein to prevent heat generation. When the rolling bearing is allowed to operate at a high rotary speed free from running-in of grease at the initial stage after filling of grease, abnormal heat generation occurs due to catching of grease or agitation resistance. Therefore, it is normally practiced to effect running-in for several times to optimize the conditions of the grease.
The recent trend is for more spindle for machine tools to operate at a higher rotary speed. It has not been seldom that the bearing for supporting the spindle is used in an operating condition where dmN (=(inner diameter of bearing+outer diameter of bearing)÷2×rotary speed (rpm)) is not lower than 1,000,000.
As compared with rolling bearings lubricated with oil air or oil mist, grease-lubricated rolling bearings tend to exhibit a reduced lifetime in high speed operation. The grease-lubricated bearings undergo seizing due to deterioration of grease before deterioration due to rolling fatigue. When the grease-lubricated rolling bearings operate at a remarkably high rotary speed, the grease deteriorates in a short period of time, causing seizing in early stage.
For example, Japanese Utility Model Laid-Open No. 1989-67331, Japanese Utility Model Laid-Open No. 1992-132220, Japanese Utility Model Laid-Open No. 1994-35659, Japanese Utility Model Laid-Open No. 1994-35653, Japanese Utility Model Laid-Open No. 1993-94531, Japanese Utility Model Laid-Open No. 1993-94532, Japanese Utility Model Laid-Open No. 1994-35655, and Japanese Utility Model Laid-Open No. 1994-35657 disclose a technique comprising a grease reservoir provided on the inner ring side and involving the continuous supply of grease by centrifugal force. Further, Japanese Utility Model Laid-Open No. 1993-86029 discloses a technique involving the effective use of grease filled in the bearing space utilizing air.
However, these techniques leave something to be desired in improvement.